The present invention relates to electrical machines and, more particularly, to machines in which high electrical stress can be generated on a surface of an insulator in the vicinity of an electrical conductor.
In a high-powered electrical apparatus such as, for example, the rotor of an electric generator, it is customary to produce slots in the surface of the generally cylindrical rotor into which field windings may be placed. In large machines, such slots may extend generally parallel to the axis of the rotor for 20 or 30 feet. Such field windings are energized during operation to produce electricity when the rotor is driven.
The rotor forging itself is generally maintained at ground potential whereas the coils or windings in the slots are maintained at relatively high potential. In order to protect against arcing from the windings to the rotor forging and also to protect against physical abrasion of the windings, it is customary to employ a slot armor lining the sides and bottom of the slot. Such slot armor is conventional and may consist of, for example, a fabric laid up from woven or non-woven glass fibers impregnated with a resin and cured by conventional means.
It is known that a high electrostatic gradient is produced at the radially outermost region of the windings adjacent to the slot armor lining the sides of the slot. This electrostatic gradient may be sufficient to produce ionization of the gaseous medium near the surface of the slot armor and thus initiate arcing along the armor surface which bridges the insulating path from the winding to the grounded rotor forging.
This problem has been recognized and met in the past by applying a grading layer to the surface of the slot armor facing the windings. The grading layer has a resistivity intermediate between that of a good insulator, in the range of 10.sup.12 ohms per square or more and that of a good conductor having a resistivity of 10.sup.-1 ohms per square or less. Such a semi-conducting material permits a limited amount of span-wise distribution of electrical charge so that the sharpness of the electrostatic gradient and consequently the likelihood of the production of a corona followed by flashover is reduced.
One type of semi-conducting material which has been in commercial use for some time is asbestos which has been applied to the surface of the slot armor. Asbestos, having a resistivity of about 10.sup.8 ohms per square and being substantially fire resistant, has performed satisfactorily in this use. The use of asbestos is no longer favored due to the possibility that it may have an adverse effect on human health.
Other types of grading materials have included a plastic material containing conducting materials, such as disclosed in U.S. Pat. No. 2,789,154 or layers of metal foil embedded within an insulating medium as disclosed in U.S. Pat. No. 2,939,976.
Certain classes of semi-conducting materials have non-linear resistance characteristics in the presence of high voltage. One such non-linear semi-conducting material is silicon carbide (SiC). In the presence of a relatively low voltage gradient, silicon carbide has a relatively high resistivity. When exposed to a high voltage gradient, on the order of, for example, several thousand volts per inch, the resistivity of silicon carbide is substantially reduced. This effect has been employed in a ceramic version of silicon carbide for lightning suppression. The use of silicon carbide in a grading system has been disclosed in U.S. Pat. Nos. 3,066,180 and 3,210,461.
Another disclosure of the use of non-linear resistive effects is to be found in U.S. Pat. No. 3,670,192 which employs a conducting layer on the ends of coils which is connected to ground through a variable non-linear resistor.
Silicon carbide, although having desirable non-linear resistance characteristics in the proper range for use in a grading system, is well known for being a very hard abrasive material. If a silicon carbide layer were incorporated in the surface of a slot armor contacting the windings, abrasion of the windings is likely to occur during motion of the windings under electrical and thermal stress.